The scale on a map is 1cm : 7km. If the distance between two cities is 63km, how far apart in cm are the two cities on the map?
Explanation: The scale means that every 1cm on the map corresponds to an actual distance of 7km. An actual distance of 63km is the same as 9 $\cdot$ 7km. The distance between the two cities on the map is 9 $\cdot$ 1cm, or 9cm.